Acoustic resonators used in radio frequency (RF) filters, such as surface acoustic wave (SAW) filters and bulk acoustic wave (BAW) filters, typically have a negative temperature coefficient of frequency (TCF) that is caused by a decrease of stiffness of materials when temperature increases. Acoustic velocity decreases with temperature and hence a filter's transfer function shifts toward lower frequencies. There are very few materials that show an irregular behavior in this regard. One example is amorphous silicon oxide. The introduction of amorphous silicon oxide to the propagation path of acoustic waves in a SAW or BAW filter may have a temperature-compensating effect and reduce the overall temperature drift of these devices. However, amorphous silicon oxide also introduces various challenges.
Amorphous silicon oxide introduces additional propagation loss, and may thwart the objective of achieving low insertion loss in filters. Furthermore, any additional material introduced into a propagation path of an acoustic wave will reduce a coupling coefficient of a resonator, which relates to the efficiency at which the resonator will convert energy between an acoustic wave form and an electrical form. As a consequence, a maximum relative filter bandwidth that a certain piezo-material can provide may decrease steeply.